The present invention relates to polishing pads for chemical mechanical planarization (CMP), and in particular, relates to polishing pads having windows formed therein for performing optical end-point detection.
In the fabrication of integrated circuits and other electronic devices, multiple layers of conducting, semiconducting and dielectric materials are deposited on or removed from a surface of a semiconductor wafer. Thin layers of conducting, semiconducting, and dielectric materials may be deposited by a number of deposition techniques. Common deposition techniques in modern processing include physical vapor deposition (PVD), also known as sputtering, chemical vapor deposition (CVD), plasma-enhanced chemical vapor deposition (PECVD), and electrochemical plating (ECP).
As layers of materials are sequentially deposited and removed, the uppermost surface of the wafer becomes non-planar. Because subsequent semiconductor processing (e.g., metallization) requires the wafer to have a flat surface, the wafer needs to be planarized. Planarization is useful in removing undesired surface topography and surface defects, such as rough surfaces, agglomerated materials, crystal lattice damage, scratches, and contaminated layers or materials.
Chemical mechanical planarization, or chemical mechanical polishing (CMP), is a common technique used to planarize substrates, such as semiconductor wafers. In conventional CMP, a wafer carrier is mounted on a carrier assembly and positioned in contact with a polishing pad in a CMP apparatus. The carrier assembly provides a controllable pressure to the wafer, urging it against the polishing pad. The pad is optionally moved (e.g., rotated) relative to the wafer by an external driving force. Simultaneously therewith, a chemical composition (“slurry”) or other fluid medium is flowed onto the polishing pad and into the gap between the wafer and the polishing pad. The wafer surface is thus polished and made planar by the chemical and mechanical action of the pad surface and slurry.
An important step in planarizing a wafer is determining an end-point to the process. Accordingly, a variety of planarization end-point detection methods have been developed, for example, methods involving optical in-situ measurements of the wafer surface. The optical technique involves providing the polishing pad with a window to select wavelengths of light. A light beam is directed through the window to the wafer surface, where it reflects and passes back through the window to a detector (e.g., a spectrophotometer). Based on the return signal, properties of the wafer surface (e.g., the thickness of films) can be determined for end-point detection.
Birang et al., in U.S. Pat. No. 6,280,290, discloses a polishing pad having a window in the form of a polyurethane plug. The pad has an aperture and the window is held in the aperture with adhesives. Unfortunately, these prior art windows have light transmission properties that hamper effective endpoint detection or measurement for a wide variety of planarizing conditions. This is due, in part, to the high degree of crystallinity of aromatic diisocyanate-based materials, such as, toluene diisocyanate (TDI), diphenylmethane (MDI) and its derivatives. These aromatic diisocyanates (TDI, MDI) are the two most commonly used in polyurethane manufacture. Furthermore, the use of aromatic diamine curatives, such as methylene bis 2-chloroaniline (MBOCA), increase crystallinity. Also, curatives such as MBOCA are colored, typically yellow to green, and impart a color to (i.e., cause absorption in) the finished polymer.
For example, typical prior art windows provide only about 50% transmission at 450 nm, and to just over 40% at 430 nm. At 400 nm, the transmission steeply declines to about 13% making robust in-situ endpoint detection or measurement difficult. This is particularly problematic due to the demand for shorter wavelength endpoint detection requirements (e.g., at 400 nm).
Hence, what is needed is a polishing pad and method for robust end-point detection or measurement during CMP over a wide range of wavelengths, and in particular, the shorter wavelengths. Also, there is a need for a polishing pad and method that can reduce the use of curatives.